Graphene, the two-dimensional, sp2-bonded allotrope of carbon, has attracted significant interest for a wide variety of applications. While a number of methods exist for the production of graphene, solution-phase exfoliation is well-suited for low-cost, large-volume applications such as printed electronics. For these applications, a stable, high-concentration dispersion of graphene in suitable solvents is of critical importance. Although reduced graphene oxide can meet this criterion, the degradation of the material during oxidation and often harsh processing steps required for reduction motivates the use of pristine graphene. The dispersion of pristine graphene typically requires select organic solvents, surfactants, or polymer stabilizers to mitigate flake aggregation.
Printed and flexible electronics are a potentially disruptive technology, offering low-cost, large-area functional devices for novel applications in areas such as environmental and biological monitoring, energy harvesting and storage, flexible displays and radio frequency identification. Solvent-exfoliated graphene is particularly well-suited for this emerging field due to its high conductivity, excellent chemical and environmental stability, inherent flexibility, and low cost. Recent research has demonstrated inkjet printing of graphene for the fabrication of functional devices such as chemical and thermal sensors, acoustic actuators, dipole antennas, and thin film transistors. While inkjet printing offers an additive technique that is ideal for rapid prototyping in research laboratories, its limited throughput motivates the development of alternative printing strategies for industrial-scale applications. Gravure printing is a promising option in this regard since it offers high-speed, roll-to-roll deposition of functional materials at high resolution.
Although several studies have demonstrated the utility of gravure printing for printed electronics, this technique has not yet been demonstrated for graphene. The absence of graphene in gravure printing can be attributed to the difficulty in formulating suitable inks since graphene possesses poor dispersion stability in common organic solvents. One strategy that enables graphene dispersions is to oxidatively exfoliate graphite, but this approach typically requires harsh chemical or thermal treatments and results in degradation of electrical properties. On the other hand, pristine graphene can be dispersed in select solvents such as N-methylpyrrolidone, but this system is unsuitable for gravure printing due to its low viscosity and limited graphene concentration. Accordingly, it remains an on-going concern in the art to develop an approach to and produce a graphene ink composition of the sort to better utilize the benefits and advantages associated with gravure printing techniques.
Screen printing is another promising approach for the practical integration of pristine graphene for applications in printed electronics. This technique is a classic mass-printing method, and is realized by pressing an ink through a patterned stencil with a squeegee. It has been widely employed for electronics because it is a versatile process, compatible with a wide variety of functional inks and substrates. Although several studies have demonstrated screen printing of reduced graphene oxide, high-resolution screen printing of pristine graphene for printed electronics has not yet been realized. This is due in part to the difficulty in producing highly loaded dispersions of pristine graphene resulting from the material's inherent tendency to aggregate. Moreover, conventional screen printing methods are restricted to a resolution of 75 to 150 μm, impeding the drive for fabrication of finer patterns to facilitate higher integration density and improved device performance. The resolution of screen printing is highly dependent on the quality of the stencil, which is generally prepared using a photochemically defined emulsion coated on a screen mesh. Although finer patterns of the stencil are expected to improve printing resolution, the low lithography resolution of the emulsion layer and mesh dimensions restricts the improvement in printing resolution. Thus, it is a challenge to develop a high-resolution stencil that can be applied to screen printing of graphene for printed electronics.